Morphology control of ultra-thin MeOx layer

ABSTRACT

A nonvolatile memory device contains a resistive switching memory element with improved device switching performance and life and methods for forming the same. The nonvolatile memory device has a first layer on a substrate, a resistive switching layer on the first layer, and a second layer. The resistive switching layer is disposed between the first layer and the second layer and the resistive switching layer comprises a material having the same morphology as the top surface of the first layer. A method of forming a nonvolatile memory element in a ReRAM device includes forming a resistive switching layer on a first layer and forming a second layer, so that the resistive switching layer is disposed between the first layer and the second layer. The resistive switching layer comprises a material formed with the same morphology as the top surface of the first layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to nonvolatile memory elements, and moreparticularly, to methods for forming resistive switching memory elementsused in nonvolatile memory devices.

2. Description of the Related Art

Nonvolatile memory elements are used in systems in which persistentstorage is required. For example, digital cameras use nonvolatile memorycards to store images and digital music players use nonvolatile memoryto store audio data. Nonvolatile memory is also used to persistentlystore data in computer environments.

Nonvolatile memory is often formed using electrically-erasableprogrammable read only memory (EEPROM) technology. This type ofnonvolatile memory contains floating gate transistors that can beselectively programmed or erased by application of suitable voltages totheir terminals.

As fabrication techniques improve, it is becoming possible to fabricatenonvolatile memory elements with increasingly smaller dimensions.However, as device dimensions shrink, scaling issues are posingchallenges for traditional nonvolatile memory technology. This has ledto the investigation of alternative nonvolatile memory technologies,including resistive switching nonvolatile memory.

Resistive switching nonvolatile memory is formed using memory elementsthat have two or more stable states with different resistances. Bistablememory has two stable states. A bistable memory element can be placed ina high resistance state or a low resistance state by application ofsuitable voltages or currents. Voltage pulses are typically used toswitch the memory element from one resistance state to the other.Nondestructive read operations can be performed to ascertain the valueof a data bit that is stored in a memory cell.

It is believed that conventional films used to form resistive switchinglayers in resistive switching nonvolatile memory provide a reversiblecreation/dissolution of a conductive filament through the resistiveswitching material between two layers serving as electrodes. Thephysical and chemical properties of the resistive switching layerdetermine the amount of power needed to switch the resistance states ofsuch films.

Moreover, since the power that can be delivered to a circuit containinga series of resistive switching memory elements and current steeringelements is typically limited in most conventional nonvolatile memorydevices (e.g., CMOS driven devices), it is desirable to form each of theresistive switching memory elements and current steering elements in thecircuit so that the voltage drop across each of these elements is small,and thus resistance of the series connected elements does not cause thecurrent to decrease to an undesirable level due to the fixed appliedvoltage (e.g., ˜2-5 volts).

As nonvolatile memory device sizes shrink, it is important to reduce therequired currents and voltages that are necessary to reliably set andreset “on” and “off” states of the device to minimize overall powerconsumption of the memory chip as well as resistive heating of thedevice and cross-talk between adjacent devices.

Moreover, as nonvolatile memory device sizes shrink it becomesincreasingly necessary to assure that the “set” and “reset” currentsused to change the state of the memory element are not so large as torequire higher voltage transistors for chip control circuitry, as wellas to minimize damage to or alter the electrical or physical propertiesof the one or more layers found in the formed memory device. A largecurrent flowing through the current carrying lines in a memory array canundesirably alter or disturb the memory state of other interconnecteddevices or possibly damage portions of the adjacent connected devices,due to an appreciable amount of “cross-talk” created between them.

There is a need to limit and/or minimize the required current used toprogram the logic states of each of the interconnected devices in aneffort to reduce chip overall power consumption as well as improvedevice longevity and reduce the possibility of cross-talk betweenadjacent connected devices, which can alter a nonvolatile memorydevice's state. It is also desirable to form a nonvolatile memory devicethat has low programming currents when switching the device between the“on” and “off” states. Therefore, it is desirable to form a nonvolatilememory device that requires low programming currents to change thedevice between the “on” and “off” states.

SUMMARY OF THE INVENTION

Embodiments according to the invention generally relate to a resistiveswitching nonvolatile memory device having a resistive switching layerand methods of forming the resistive switching layer. In someembodiments, a nonvolatile memory element includes a first layer on asubstrate, a resistive switching layer on the first layer, and a secondlayer. The resistive switching layer is disposed between the first layerand the second layer and the resistive switching layer comprises amaterial having the same morphology as the first layer.

In some embodiments, the method of forming a nonvolatile memory elementin a ReRAM device includes forming a resistive switching layer on afirst layer and forming a second layer, so that the resistive switchinglayer is disposed between the first layer and the second layer. Theresistive switching layer comprises a material having the samemorphology as the first layer.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features can be understoodin detail, a more particular description may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments and are therefore not to be considered limiting ofits scope, and may admit to other equally effective embodiments.

FIG. 1 is a perspective view of a memory array of memory devices.

FIG. 2A is a schematic view of a memory device.

FIG. 2B schematically illustrates a memory device configured to allowcurrent to flow through the memory device in a forward direction.

FIG. 2C is schematic representation of an array of memory devices.

FIG. 2D is schematic representation of another array of memory devices.

FIG. 3 schematically illustrates exemplary log-log plots of measuredcurrent (I) values versus applied voltages (V) of an exemplaryembodiment of a memory device having a resistive switching memoryelement.

FIG. 4A is a schematic side cross-sectional view of a memory elementdisposed in a nonvolatile memory device.

FIG. 4B is a schematic representation of an electrical circuit formed inthe memory element illustrated in FIG. 3A.

FIG. 5 is a process flow diagram illustrating the step for forming theswitching memory device according to one embodiment.

FIG. 6A is a schematic depiction of a resistive switching layer and anunderlying layer, each having a crystalline morphology.

FIG. 6B is a schematic depiction of a resistive switching layer and anunderlying layer, each having an amorphous morphology.

FIG. 6C is a schematic depiction of a resistive switching layer and anunderlying layer, each having a polycrystalline morphology.

FIG. 6D is a schematic depiction of a resistive switching layer and anunderlying layer, each having an intermixed morphology.

FIG. 6E is a schematic depiction of a resistive switching layer and anunderlying layer, each having an intermixed morphology.

FIG. 7 schematically illustrates a cross-sectional view of a resistiveswitching layer formed according to an embodiment.

While the foregoing is directed to embodiments described herein, otherand further embodiments may be devised without departing from the basicscope thereof, and the scope thereof is determined by the claims thatfollow.

DETAILED DESCRIPTION

Embodiments generally described herein include a method of forming anonvolatile memory device that contains a resistive switching memoryelement that has an improved device switching performance and increasedusable lifetime. Embodiments provide a resistive switching layer whosemorphology can be controlled by the morphology of the underlying layerin the nonvolatile memory element. The underlying layer may not onlyhelp control morphology of an overlying material, but it also influencesthe inter-atomic distance of the overlying material i.e. the underlyinglayer may modify the natural lattice distance of the overlying material.In the context of material science and chemistry, morphology is the formor structure of different materials on an atomic level. For example, itis well known that carbon may exist in multiple structures (allotropes)such as diamond, where carbon atoms bond together in a tetrahedrallattice (crystalline type structure), and graphite, where carbon atomsbond together in sheets of a hexagonal lattice. Different allotropes ofa material usually have different physical, electrical, optical,chemical, and other properties. In another example, materials mayexhibit polymorphism, i.e. the ability of the material to present itselfin different crystal forms and structures. Complex materials, such asmetal oxides, used for electronic applications may exhibit polymorphism.Thus, such complex materials may be formed to have certain crystallinestructures and inter-atomic lattice distances in order to providedesired electrical and physical properties.

Embodiments described herein may exhibit various types of morphologysuch as crystalline, polycrystalline, and amorphous. Some embodimentsmay exhibit an intermixed morphology. Crystalline structures aregenerally exhibited by a solid material whose constituent atoms ormolecules are arranged in an ordered pattern extending in all threespatial dimensions. The atoms and/or molecules are interconnected andcontinue uniformly over a long distance, creating a well-ordered crystallattice. If the crystal lattice extends continuously the full distancebetween the surfaces of the material (without being interrupted by grainboundaries), it may be referred to as a single-crystal (ormono-crystal). Thus, a crystalline material is made up of atoms in anorderly array.

In contrast, amorphous structures exhibit no recognizable long-rangeorder such as is exhibited in crystalline structures. Thus, theconstituent atoms and/or molecules in an amorphous material are randomlyoriented and rarely interconnected so that there is no long-rangeordering. “Ordering” is the regularity with which atoms appear inpredictable locations in a lattice, as measured from a selected point oforigin. In a perfectly-crystalline material, the location of each atomin the lattice can be determined by measuring from the point of origin.Conversely, in a more disordered structure (such as an amorphous solid),the location of the first tier (or perhaps a few tiers) of nearby atomscan be described with some degree of certainty, but the ability toaccurately predict the locations of atoms then decreases dramaticallymoving away from the point of origin. Polycrystalline structuresgenerally exhibit crystal grains separated by grain boundaries with noamorphous matrix between the crystal grains. Thus, the polycrystallinematerial is completely ordered in segments. An intermixed morphology mayhave regions of crystalline structures, such as micro-crystals,intermixed with amorphous regions. In order to detect the crystallinityof a material, a Focused Ion Beam (FIB) or other similar technique isused to cut the material. A material inspection technique, such asHigh-Resolution Transmission Electron Microscopy (HRTEM) or ScanningTunneling Microscopy (STM), is then able to show the atomic structure ofthe observed material. Other techniques like Selected Area ElectronDiffraction (SAED) reveal the crystallinity of the material by observingthe diffraction pattern. Due to the atomic resolution of thesetechniques each atomic layer can be discerned and thus the crystallinityof the material.

It is noted that relative directional terms used herein with regard toembodiments are for purposes of description only, and do not limit itsscope as understood by persons skilled in the art. Specifically,directional terms such as “over,” “above,” “under,” and the like areused in relation to a substrate upon which layers making up theembodiments described herein are formed, is a “bottom” element and istherefore “under” elements of the other layers of the embodiments formedthereon.

In some embodiments, a nonvolatile memory element includes a first layerformed on a substrate, a resistive switching layer formed on the firstlayer, and a second layer. The resistive switching layer is disposedbetween the first layer and the second layer and the resistive switchinglayer comprises a material formed with the same morphology as the firstlayer. The first layer morphology may be crystalline, polycrystalline,or amorphous and the resistive switching layer will have the samemorphology. In some embodiments, if the first layer has apolycrystalline structure (morphology), the resistive switching elementis formed on the first layer so that it will also have a polycrystallinestructure. By controlling the morphology of the resistive switchinglayer, the conductivity of the resistive switching layer and otherelectrical and physical properties can be tailored as desired. Theelectrical and physical properties of the formed resistive switchinglayer are configured to lower the power required to change the resistiveswitching layer from a low resistance state (LRS) to a high resistancestate (HRS) and vice-versa, to increase device endurance and switchingrate, and to decrease device variability. It is generally desirable toform the resistive switching layer so that its material and electricalproperties will not degrade or breakdown during the often high current“burn-in” type device preparation steps, such as the “electricalforming” process, and also during normal repetitive operation of thenonvolatile resistive switching memory device. The general optimizationof ReRam device performances may be addressed by the morphology controltechnique.

An illustrative memory array 100 of nonvolatile resistive switchingmemory devices 200 (hereafter switching memory device 200), which eachgenerally include at least one resistive switching memory element 112,is illustrated in FIG. 1. Memory array 100 may be part of a largermemory device or other integrated circuit structure, such as a system ona chip type device. Read and write circuitry is connected to switchingmemory devices 200 using word-lines and bit-lines, which are referred toherein generally as electrodes 102 and 118, and are used to read from orwrite data into the memory devices 200. Electrodes 102 and 118 generallyinclude one or more conductive layers, each conductive layer having adesired function in the array of switching memory devices 200. In someconfigurations, the electrodes 102 and 118 each comprise two or moreconductive layers in which a first conductive layer is used tointerconnect the multiple switching memory devices 200 and a secondconductive layer is disposed in each switching memory device 200 toprovide a desirable electrical interface (e.g., desirable work function)to the adjacent components in the switching memory device 200.Individual switching memory devices 200 or groups of switching memorydevices 200 can be accessed using appropriate sets of word-lines andbit-lines, or electrodes 102 and 118. The memory elements 112 in theswitching memory devices 200 may be formed from one or more layers 114of materials, as indicated schematically in FIG. 1. In addition, memoryarrays such as memory array 100 can be stacked in a vertical fashion tomake multilayer memory array structures. The use of resistive switchingmemory elements to form memory arrays is merely illustrative, and oneskilled in the art will appreciate that the formed devices may be usedin other device applications without deviating from the basic scopedescribed herein.

FIG. 2A schematically illustrates an example of some embodiments of aswitching memory device 200 that contains a memory element 112 and anoptional current steering device 216, which are both disposed betweenthe electrodes 102 and 118. In some embodiments, the current steeringdevice 216 is an intervening electrical component, such as a p-njunction diode, p-i-n diode, transistor, or other similar device that isdisposed between electrode 102 and memory element 112, or between theelectrode 118 and memory element 112. In some embodiments, the currentsteering device 216 may include two or more layers of semiconductormaterial, such as two or more doped silicon layers, that are configuredto allow or inhibit the current flow in different directions through thememory element 112 when that memory element is dormant, i.e. notselected to read.

FIG. 2B schematically illustrates another switching memory device 200′that contains a memory element 112 and a diode type current steeringdevice 216′ that preferentially allows current to flow through thememory device 200 in a forward direction (“I⁺”). However, due to thedesign of the current steering device 216′, a reduced current can alsoflow in the opposing direction through the device by the application ofa reverse bias to the electrodes 102 and 118.

FIG. 2C schematically illustrates an array of switching memory devices200 that are connected together to form part of a high-capacitynonvolatile memory integrated circuit. Each of the individual switchingmemory devices 200 can be accessed using appropriate sets of discreteword-lines and bit-lines, which, as noted above, may comprise at least aportion of the electrodes 102 and 118. As illustrated in FIG. 2C, eachof the switching memory devices 200 contains a memory element 112 andcurrent steering device 216 (e.g., a diode type) that are connected toat least one of the electrodes 102 and at least one of the electrodes118. The electrodes 102 and/or 118 are generally biased by circuitrythat is connected at the periphery of the memory chip on which the arrayof memory devices 200 is formed.

FIG. 2D schematically illustrates an example of some embodiments of anarray of switching memory devices 200 that are connected together toform part of a high-capacity nonvolatile memory integrated circuit. Asshown in FIG. 2D, the current steering device 216, such as a typical MOStype transistor, is used to selectively deliver current through thememory element 112 by use of the appropriate set of word-lines,bit-lines and separate source-lines 119. As illustrated in FIG. 2D, eachof the switching memory devices 200 contains a memory element 112 andcurrent steering device 216 (e.g., transistor) that are connected to atleast one of the electrodes 102, at least one of the electrodes 118 andat least one of the source lines 119. The source-lines 119 generallycomprise one or more patterned conductive layers (e.g., metal) that areadapted to provide a desired amount of current to the memory element 112when the transistor in the current steering device is turned “on”. Theelectrodes 102, electrodes 118 and source-lines 119 are typically biasedby circuitry that is connected at the periphery of the memory chip onwhich the array of memory devices 200 is formed.

During operation, such as a read operation, the state of a memoryelement 112 in the switching memory device 200 can be sensed by applyinga sensing voltage (i.e., a “read” voltage V_(READ) (FIG. 3)), such asapplying about +0.5 volts (V), to an appropriate set of electrodes 102and 118. Depending on its history, a memory element that is addressed inthis way may be in either a high resistance state (HRS) or a lowresistance state (LRS). The resistance of the memory element 112therefore determines what digital data is being stored by the memoryelement 112. If the memory element 112 is in the high resistance state,for example, the memory element may be said to contain a logic “zero”(i.e., a “0” bit). If, on the other hand, the memory element is in thelow resistance state, the memory element may be said to contain a logic“one” (i.e., a “1” bit). During a write operation, the state of a memoryelement can be changed by application of suitable write signals to anappropriate set of electrodes 102 and 118.

In some embodiments, the memory element 112 uses bipolar switching whereopposite polarity set and reset voltages are used to alter theresistance of the memory element between high and low resistance states.FIG. 3 schematically illustrates a log-log plot of current (I) versusvoltage (V) of one example of the low-resistance-state (LRS) andhigh-resistant-state (HRS) curves of a memory element 112 having thesetwo bistable resistive states. The voltage levels V_(SET) and V_(RESET),shown in FIG. 3, illustrate typical threshold values used to “set” and“reset” the contents of a memory element 112. In one example, initially,memory element 112 may be in a high resistance state (e.g., storing alogic “zero”). The high resistance state of memory element 112 can besensed by read and write circuitry 150 (FIG. 2A) using electrodes 102and 118. For example, read and write circuitry may apply a read voltageV_(READ) to memory element 112, and can sense the resulting “off”current (I_(OFF)) that flows through memory element 112. When it isdesired to store a logic “one” in memory element 112, memory element 112can be placed into its low-resistance state. This may be accomplished byusing read and write circuitry 150 to apply a set voltage V_(SET) (e.g.,−1 V to −4 V) across electrodes 102 and 118. In one configuration,applying a negative V_(SET) voltage to memory element 112 causes memoryelement 112 to switch to its low resistance state. It is believed thatthe change in the resistive state of memory element 112 may be due tothe redistribution or filling of traps (i.e., “trap-mediated”) ordefects in the resistive switching layer 206 (FIG. 4, 6A, or 8A) ofmemory element 112 when the memory device 200 is reverse biased. Thedefects or traps, which are commonly thought to be oxygen vacancies, maybe formed during the deposition and/or post-processing of the resistiveswitching layer 206. For example, the defects may be created by anon-stoichiometric material composition found in the formed resistiveswitching layer 206. In embodiments in which a novel resistive switchinglayer comprises a metal nitride or metal oxide-nitride, it is believedthat the defects or traps that provide multiple resistance capabilityfor the resistive switching layer may be nitrogen vacancies.

The low resistance state of the memory element 112 can be sensed usingthe read and write circuitry 150. When a read voltage V_(READ) isapplied to resistive switching memory element 112, the read and writecircuitry 150 will sense the relatively high “on” current value(I_(ON)), indicating that memory element 112 is in its low resistancestate. When it is desired to store a logic “zero” in memory element 112,the memory element can once again be placed in its high resistance stateby applying a positive reset voltage V_(RESET) (e.g., +1 V to +5 V) tomemory element 112. When read and write circuitry applies V_(RESET) tomemory element 112, memory element 112 enters its high resistance state.When the reset voltage V_(RESET) is removed from memory element 112,memory element 112 will once again be characterized by high resistancewhen the read voltage V_(READ) is applied. Voltage pulses can be used inthe programming of the memory element 112. For example, a 1 microseconds(μs) to 1 nanoseconds (ns) square or trapezoidal shaped pulse can beused to switch the memory element 112. In some embodiments, it may bedesirable to adjust the length of the pulse depending on the amount oftime needed to switch the memory element 112. In one example, the “set”and “reset” pulses are each about 10 ns in length. While the discussionof the memory element 112 herein primarily provides bipolar switchingexamples, some embodiments of the memory element 112 may use unipolarswitching, where the set and reset voltages have the same polarity.

FIG. 4A is a schematic cross-sectional view of an un-optimized switchingmemory device 200, which contains a memory element 112 and a currentsteering device 216 (e.g., a diode) that is formed over a substrate 201.In this configuration, the memory element 112 generally contains a topelectrode 102, resistive switching layer 206, and intermediate electrode210. FIG. 4B schematically illustrates an electrical circuit formed inthe switching memory device 200 shown in FIG. 4A. As illustrated in FIG.4B, the electrical circuit within the standard switching memory device200 includes a top electrode impedance (i.e., resistance R_(TEL))created by the material layer(s) in the top electrode 102, a resistiveswitching layer impedance (i.e., resistance R_(RSL)) created by thematerial layer(s) in the resistive switching layer 206, an intermediateelectrode impedance (i.e., resistance R_(IEL)) created by the materiallayer(s) in the intermediate electrode 210, a current steering deviceimpedance (i.e., resistance R_(CSD)) created by the material layer(s) inthe current steering device 216 and a bottom electrode impedance (i.e.,resistance R_(BEL)) created by the material layer(s) in the bottomelectrode 118. The electrodes 102, 210 and 118 are generally formed froma conductive material, such as a highly conductive semiconductormaterial (e.g., p-type polysilicon, n-type polysilicon) and/or metal(e.g., TiN, Al, W) to minimize the circuit resistance created betweeninterconnected devices in a memory array 100. The resistive switchinglayer 206 can be a dielectric material, such as a metal oxide materialor other similar material that can be switched between at least two ormore stable resistive states. In some embodiments, the high and lowresistance states of the resistive switching layer 206 may be about 2.5MΩ and about 100 kΩ, respectively. One will note that it is assumed thatthe contact resistances between the various layers in the switchingmemory device, such as the contact resistance formed between theelectrode 102 and the resistive switching layer 206, are negligible tohelp reduce the complexity of the discussion of the circuit. While thecurrent steering device 216 may include two or more layers ofsemiconductor material that are adapted to control the flow of currentthrough the formed memory device 200, the resistance of each of thecomponents in the current steering device 216 are not individuallydiscussed herein to minimize the complexity of the discussion, and thusan overall current steering device resistance R_(CSD) is used torepresent the overall impedance of the current steering device 216.

Embodiments described provide a resistive switching layer 206 formed ona first layer, which first layer is formed on a substrate 201. Theresistive switching layer 206 comprises a material having the samemorphology as the layer on which it is formed. By controlling themorphology of the resistive switching layer 206, such as by forming theresistive switching layer 206 to be an ultra-thin layer on an electrode,the conduction of the resistive switching layer 206 can be controlled.The resistive switching layer 206 is disposed between the first layerand the second layer, wherein the resistive switching layer comprises amaterial formed with the same morphology as the first layer.

In general the conduction properties of a material are related to themorphology of the material, such that in a film with long order range,conduction is easier compared to a film with short order range or noorder at all, assuming similar film thicknesses. For example, theconduction in the grain boundaries in between poly-crystals can beassisted by the highly defective paths so that they become the mainconductive regions. A perfect crystalline material can give origin to asemiconductor behavior while an amorphous material can give origin to aninsulator behavior (of course the presence of defects in an amorphousmaterial can create a conduction band like in a semiconductor conductorbut with different conduction properties). Additionally, diffraction,reflectivity, transmission, absorption and pairs generation propertiesare related to the morphology of the material.

FIG. 5 is a process flow diagram of a process 500 (method) for formingthe switching memory device 200. Although the method steps are describedin conjunction with memory device 200 in FIG. 4A, persons skilled in theart will understand that other resistive switching memory devices may beformed using process 500. It is also noted that the process for formingthe current steering device is not shown in FIG. 5.

Process 500 begins at step 502 in which an electrode is formed on orover substrate 201, which may be electrode 118 or intermediate electrode210. In one embodiment the first layer is an electrode, such as thebottom electrode 118 or intermediate electrode 210 shown in FIG. 4A. Theelectrodes 102, 210, and 118 are generally formed from a conductivematerial, such as a highly conductive semiconductor material (e.g.,p-type polysilicon, n-type polysilicon) and/or metal (e.g., TiN, Al, W).In one embodiment, electrodes 118 or 210 are a highly doped polysiliconlayers that are formed on or over substrate 201 using a conventional CVDor ALD type polysilicon deposition technique. In one embodiment,electrode 118 and/or electrode 210 comprises polysilicon, and is betweenabout 50 and about 5000 Å thick. The first layer has a morphology thatis crystalline, polycrystalline, or amorphous. The morphology of thefirst layer may be controlled in different ways. In some embodiments, ifthe electrode 118 is a thin layer, its morphology is determined by themorphology of the underlying substrate 201. However, if the electrode118 is thick enough to be independent from the morphology of theunderlying substrate 201, then its morphology is controlled by thedeposition technique parameters (temperature, speed, precursors, etc.)and by the kind of material used for the electrode 118. For example,pure single-crystal materials like silicon or copper can be obtainedunder particular deposition techniques, e.g. Czochralski and/or Bridgmanprocesses. Polysilicon is generally made of polycrystals whosepercentage and size can be controlled during the deposition process.Generic metals like TiN and Al generally have a polycrystallinemorphology with cubic crystal structure. A special class of metals isthe so called metallic glass, characterized by an amorphous structure.

In step 504, resistive switching layer 206 is formed on or over anelectrode, such as electrode 118 or intermediate electrode 210, usingone or more deposition processes. Resistive switching layer 206comprises a dielectric material that can be switched between two or morestable resistive states. In some embodiments, resistive switching layer206 is an ultra-thin layer that has a thickness of less than about 5nanometers (nm), such as from about 2 nm to about 3 nm. Conventionalresistive switching layers are generally between 20 to 30 nm thick, thussome embodiments provide resistive switching layers that generally maybe an order of magnitude thinner than conventional films.

It is believed that for ReRAM devices, the morphology of the switchinglayer can control the mobility of the oxygen vacancies that are mostlikely responsible for the creation and/or dissolution of the conductionpath between the electrodes and through the switching layer. In general,the ability to control the morphology, and thus the mobility of oxygenvacancies of a material, has an impact on electron conduction and ionconduction through the material. Moreover, the morphology can controlthe heat diffusion into the surrounding area of the switching event.This can be a very important property due to the fact that all thephysical and/or chemical mechanisms at the origin of theresistive-switching are thermally activated. For these reasons, controlof the material morphology can be considered another “knob” that can beused to tailor the switching performances in various memory devices.

Controlling the morphology of a layer may have other applications inaddition to switching layers in nonvolatile memory devices. In the caseof a resistor, the morphology can be tuned to not only have the desiredresistivity, but to also have the desired mechanism of conduction. Inthe case of a capacitor, the morphology can help minimize the currentleakage. In the case of a power device, controlling the morphology canhelp toward better power dissipation. In the case of CBRAM devices, theconduction path is also created by the movement of high mobility ionscoming from one active electrode (e.g. Cu or Ag) into a solid-stateelectrolyte. Thus, controlling the morphology of layers in CBRAM devicesmay also improve the ability to control ion mobility as desired.

The morphology of the switching layer can control the mobility of theoxygen vacancies that are most likely responsible for the creationand/or dissolution of the conduction path between the electrodes andthrough the switching layer. It is believed that a similar mechanism canbe considered in CBRAM devices in which the conduction path is createdby the movement of high mobility ions coming from one active electrode(e.g. Cu or Ag) into a solid-state electrolyte.

The resistive switching layer 206 may be formed using CVD, ALD, orsimilar processes that provide precise layer control. During theformation process, the resistive switching layer 206 may be formed bydepositing one or more layers on the electrode on the order of a singlemonolayer in thickness. The first monolayer of the resistive switchinglayer 206 will match the morphology of the underlying layer. Eachsubsequent monolayer will also match the morphology of its underlyinglayer at least up to a certain thickness. If the resistive switchinglayer becomes too thick, then it will have a morphology that may bedifferent than and independent of the underlying first layer. Thus, anultra-thin resistive switching layer will have the same morphology asthe underlying layer. Substantially the entire resistive switching layerhas the same morphology as the underlying layer.

Numerous materials have been explored for possible use in resistiveswitching layers for nonvolatile resistive memory devices, includingvarious oxides and all of the transition metals, e.g., aluminum (Al),hafnium (Hf), zirconium (Zr), titanium (Ti), tantalum (Ta), nickel (Ni),copper (Cu), tungsten (W), iron (Fe), vanadium (V), niobium (Nb),tungsten (W), and the like. Some examples of suitable metal oxides foruse as a resistive switching layer 206 include AlO_(x), HfO_(x), andTiO_(x).

Resistive switching layer 206 may also include a two-metal oxide, ametal nitride, a metal oxide-nitride, a two-metal oxide-nitride, or amultilayer stack thereof. In general, materials like phase changechalcogenides, perovskite, solid-state electrolytes, and organiccompounds whose layer is similar to a poly-crystalline type may also beused.

Metal nitrides suitable for use as a resistive switching layer 206include HfN_(x), ZrN_(X), SiN_(X), AlN_(X), TiN_(X), V_(X)N_(Y) (e.g.,V₂N), NbN_(X) (e.g. Nb₂N, Nb₄N₃, NbN), WN_(x) (e.g., WN₂), among others.Suitable metal nitrides can be formed by atomic layer deposition (ALD)processes.

Metal oxide-nitrides suitable for use as resistive switching layer 206include HfO_(x)N_(y), ZrO_(x)N_(y), AlO_(x)N_(y), and TaO_(x)N_(y),among others. As used herein, the term “metal oxide-nitride” refers to amaterial that is a combination of metal oxides and metal nitrides, incontrast with metal oxynitrides, in which the metallic chemical elementis bonded with the ON structure. According to embodiments of theinvention, such metal oxide-nitride films can be deposited in variousALD processes that are described below in conjunction with FIGS. 6A-9.

Two-metal oxide-nitrides suitable for use as resistive switching layer206 have two metallic or semi-metallic elements and include films suchas: Hf_(x)Si_(y)O_(z)N_((1-x-y-z)), Zr_(x)Si_(y)O_(z)N_((1-x-y-z)),Hf_(x)Zr_(y)O_(z)N_((1-x-y-z)), and Hf_(x)Al_(y)O_(z)N_((1-x-y-z)).Two-metal oxide-nitrides may be deposited by a series of ALD processessimilar to that used for the deposition of metal oxide-nitrides. SuchALD processes are described below in conjunction with FIGS. 6A-9.Suitable metals for such two-metal oxide-nitrides include hafnium (Hf),zirconium (Zr), aluminum (Al), tantalum (Ta), titanium (Ti), vanadium(V), niobium (Nb), and tungsten (W) and suitable semi-metallic elementsinclude silicon (Si).

As previously stated, the resistive switching layer has the samemorphology as the first layer. It is believed that this is achieved byforming the resistive switching layer as an ultra-thin layer having athickness less than about 5 nm. In the example of some embodiments shownin FIG. 4A, where the first layer is the intermediate electrode 210, theresistive switching layer 206 has the same morphology as theintermediate electrode 210. In some embodiments, if the intermediateelectrode 210 comprises a crystalline doped polysilicon, then theresistive switching layer 206 also is crystalline. If the intermediateelectrode 210 comprises a TiN material having a polycrystallinemorphology, then the resistive switching layer 206 also comprises amaterial having a polycrystalline morphology.

FIGS. 6A-6E illustrate the different morphologies the resistiveswitching layer and the first layer may have. FIG. 6A schematicallyillustrates the first layer and the resistive switching layer at anatomic level. In this example of some embodiments, the first layer 612 ahas a crystalline structure 600 a. As the resistive switching layer 610a, shown in the darker shading, is formed thereon, it will match themorphology of the underlying layer and also have a crystalline structure600 a. Although both layers have a crystalline structure, thecrystalline structure of the resistive switching layer 610 a may or maynot have the same inter-atomic distance as the underlying crystallinelayer, i.e. the first layer 612 a. Generally, the inter-atomic distanceis considered to be the distance between atoms in a lattice structure.Thus, although both layers may be crystalline, they may have latticestructures with different interatomic distances. Even when thecrystalline structures of the two layers have differing interatomicdistances in their lattice structures, their morphologies are stillconsidered to be the same as each layer has a crystalline morphology.FIG. 6B schematically illustrates an amorphous structure 600 b where theresistive switching layer 610 b, shown in the darker shading has thesame amorphous morphology as the first layer 612 b as it is formedthereon. As such, both the first layer 612 b and the resistive switchinglayer 610 b have an amorphous structure. FIG. 6C schematicallyillustrates a polycrystalline structure 600 c. In this example of someembodiments, the resistive switching layer 610 c, shown in the darkershading, and the first layer 612 c each have a polycrystallinestructure. Dashed lines 614 c represent the grain boundaries betweenindividual crystals in the polycrystalline structure 600 c. Themorphologies of the first layer 612 c and the resistive switching layer610 c are the same in that the grain boundaries in the polycrystallinestructure of the first layer 612 c match the grain boundaries formed inthe polycrystalline structure of the resistive switching layer 610 c.

FIGS. 6D and 6E illustrate some embodiments having layers exhibitingintermixed morphological structures. In some embodiments, the firstlayer and the resistive switching layer may have portions that exhibit acrystalline structure mixed with portions that exhibit an amorphousstructure. Embodiments having an intermixed morphology may be describedas having a degree of crystallinity, such as 80% crystalline.Crystallinity of a film expressed as a percentage indicates that someportions of the layer have amorphous regions intermixed with crystallineregions. Generally, the percentage of crystallinity is an average numberof crystalline structures present in a fixed area/volume of the film.

FIG. 6D illustrates an example of some embodiments of an intermixedmorphology 600 d, portions of which exhibit an amorphous structure 602 dand portions of which exhibit a crystalline structure 604 d. In thisexample, the resistive switching layer 610 d, shown in the darkershading, and the first layer 612 d each have portions that are amorphous602 d and portions that are crystalline 604 d. Dashed lines 614 drepresent the grain boundaries between the amorphous structure 602 d andthe crystalline structure 604 d of the intermixed morphology 600 d. Themorphologies of the first layer 612 d and the resistive switching layer610 d are the same in that the grain boundaries in the intermixedstructure 600 d of the first layer 612 d match the grain boundariesformed in the intermixed structure of the resistive switching layer 610d. Additionally, when the degree of crystallinity is the same for thefirst layer 612 d and the resistive switching layer 610 d, as shown inFIG. 6 d, then the first layer 612 d and resistive switching layer 610 dhave the same morphology.

FIG. 6E illustrates an example of some embodiments of an intermixedmorphology 600 e, portions of which exhibit an amorphous structure 602 eand portions of which exhibit a crystalline structure 604 e. In thisexample, the resistive switching layer 610 e, shown in the darkershading, and the first layer 612 e each have portions that are amorphous602 e and portions that are crystalline 604 e. Dashed lines 614 e and616 e represent the grain boundaries between the amorphous structure 602e and the crystalline structure 604 e of the intermixed morphology 600e. This example also illustrates a portion of the first layer 612 eexhibiting an amorphous structure 602 e, as shown in the region betweenthe two grain boundaries 614 e and above the grain boundary 616 e. Thisamorphous structure 602 e of the first layer 612 e is above acrystalline structure 604 e of the first layer 612 e, as shown in theregion between the two grain boundaries 614 e and below the grainboundary 616 e. In this example, the switching layer 610 e exhibits thesame amorphous structure 602 e as the first layer 612 e, as shown in theregion between the two grain boundaries 614 e and above the grainboundary 616 e. The morphology of the resistive switching layer 610 ewill have the same morphology as that of the underlying first layer 612e at its top surface, despite that the underlying first layer 612 e mayhave a different morphology below the top surface of the first layer 612e, as illustrated in FIG. 6E. The top surface of the first layer 612 emay be one or two atomic layers thick. The top surface may also becharacterized as the portion of the underlying layer where the gases ofthe ALD cycles react.

In an intermixed ultra-thin film layer, the morphology of the both firstlayer and the resistive switching layer may be determined by usingmaterial inspection techniques, for example comparing an XRD analysis ofthe ultra-thin film layer with an XRD analysis of the portion of firstlayer that is adjacent the ultra-thin film layer. In each of theseexamples of some embodiments, the resistive switching layers 610 a, 610b, 610 c, 610 d, 610 e may be a metal oxide while the first layers 612a, 612 b, 612 c, 612 d, 612 e may be the underlying electrode.

The different morphologies create different physical and electricalproperties in the resistive switching layer 206. For example,crystalline structures have a certain kind of conductivity, whilepolycrystalline structures provide conductivity through grainboundaries. The first layer may be selected to provide a particularmorphology to the resistive switching layer 206. Generally, if a ReRAMdevice requiring lower power and lower variability is desired, acrystalline structure may be chosen because of its ordered and welldefined structure to likely provide lower power and lower variability.During device formation, it may be necessary to apply certain voltagesto make a conductive path through the resistive switching layer. Theamount of voltage required to form the conductive path may vary fromdevice to device, creating device variability. Thus, a crystallinestructure may lower the variability in performance and programming ofthe device. A polycrystalline structure may be selected if a ReRAMdevice providing better retention and lower power requirements isdesired. However, polycrystalline structures may also increaseresistive-switching variability related to the presence of multipleconductive paths in parallel, i.e. along the grain boundaries. Anamorphous material may provide a better thermal confinement, thuslowering the energy barrier for the resistive-switching process. Theparticular morphology may also be chosen to improve the number of cyclesa device may cycle between resistive states before failure.

In step 506, electrode 102 is formed above resistive switching layer 206as shown in FIG. 4A using one or more of the materials suitable forelectrode 102 listed above in conjunction with FIG. 4A. Electrode 102may be formed using a deposition process, such as PVD, CVD, ALD or othersimilar processes. In one embodiment, electrode 102 is between about 500Å and 1 μm thick.

In optional step 508, formed memory device 200 is thermally processed,e.g., via an anneal process. Temperature and duration of the annealprocess is a function of the configuration of memory device 200 as wellas the materials included in memory device 200. For example, in someembodiments, the anneal process takes place at a temperature of greaterthan about 200° C. In other embodiments, the anneal process takes placeat a temperature of greater than about 600° C. In yet other embodiments,the anneal process takes place at a temperature of greater than about1000° C. Duration of the anneal process can also vary greatly, e.g.varying between about 30 seconds and 20 minutes depending on theconfiguration of memory device 200. Furthermore, vacuum anneals, oxygenanneals, anneals using gas mixtures, such as a hydrogen/argon mixture,and other anneal processes known in the art fall within the scope of theinvention. Similarly, multiple thermal processing steps may be performedon memory device 200. In some embodiments, a thermal process may beperformed during or after multiple steps of method 500. The process(es)performed at step 508, are generally configured to cause the layersdisposed in the switching memory device 200 to form a desirableinterface between adjacent layers as well as activate and/or desirablyprocess the other layers formed in the switching memory device.

FIG. 7 schematically illustrates a cross-sectional view of a resistiveswitching layer formed according to some embodiments. The resistiveswitching layer 206 in a nonvolatile memory device 200 may comprisemultiple sub-layers as shown in FIG. 7. Resistive switching layer 206 isformed on the first layer, such as electrode 118, and the second layer,such as electrode 102, is formed so that the resistive switching layer206 is disposed between the first layer and the second layer.

The resistive switching layer 206 may comprise two or more sub-layers701, 702 having different morphologies. The first sub-layer 701comprises a material formed with the same morphology as the first layer,such as electrode 118. The second sub-layer 702 comprises a materialformed with a morphology that is either the same as or different thanthe first sub-layer 701 and the first layer, such as electrode 118. Insome embodiments, the first layer (e.g. electrode 118) and the firstsub-layer 701 may have a polycrystalline morphology and a thickness ofabout 5 nm or less. The second sub-layer, however, may have apolycrystalline or a crystalline morphology, and a thickness of about 30nm or less. In some embodiments, the second sub-layer 702 may be 5 nm orless so that it will have the same structure of sub-layer 701 e.g. bothsub-layers 701 and 702 are polycrystalline. In some embodiments, thesub-layer 702 may be thick enough to form a crystalline structure thatis independent from sub-layer 701, e.g. around 30 nm, although theactual thickness required to form an independent sub-layer will dependon the material used. The first sub-layer 701 and second sub-layer 702may comprise the materials previously discussed in relation to theresistive switching layer 206, although each sub-layer comprisesdifferent materials in order to form each sub-layer with differentmorphologies. The morphology control of these two layers provides anextra “knob” to improve switching performance.

Embodiments described provide the ability to tailor the electricalproperties and switching characteristics of a resistive switching layer.A resistive switching layer is formed as an ultra-thin layer to have amorphology matching the underlying layer, such as an electrode. Byproviding a resistive switching layer with the same morphology of theunderlying layer the resistive switching layer may be easier to switchand/or provide better resistive state retention compared to conventionalswitching films. Additionally, the films formed according to embodimentsdescribed may be formed entirely in-situ (in one processing chamber),thereby simplifying processing steps and decreasing processing times.Moreover, as memory devices continue to decrease in size, ultra-thinresistive switching layers provide a means of scaling memory devices toachieve smaller sizes.

The foregoing embodiments are merely illustrative of the principles andvarious modifications that can be made by those skilled in the artwithout departing from the scope and spirit of the invention as definedby the claims that follow.

The invention claimed is:
 1. A nonvolatile memory element, comprising: afirst layer on a substrate; a resistive switching layer on the firstlayer; a second layer; wherein the resistive switching layer is disposedbetween the first layer and the second layer, and wherein the resistiveswitching layer comprises a material having the same morphology as a topsurface of the first layer; a first sub-layer that comprises thematerial having the same morphology as a top surface of the first layer;and a second sub-layer comprising a material having a morphology that isdifferent than the first sub-layer and the first layer, wherein thefirst layer and the first sub-layer each have a polycrystallinemorphology and the second sub-layer has a crystalline morphology.
 2. Thenonvolatile memory element of claim 1, wherein the morphology of thefirst layer and the resistive switching layer is crystalline,polycrystalline, amorphous, or intermixed.
 3. The nonvolatile memoryelement of claim 1, wherein the resistive switching layer has athickness of about 5 nanometers (nm) or less.
 4. The nonvolatile memoryelement of claim 3, wherein the resistive switching layer has athickness from about 2 nm to about 3 nm.
 5. The nonvolatile memoryelement of claim 1, wherein the resistive switching layer comprises ametal oxide layer, a two-metal oxide layer, a metal nitride layer, ametal oxide-nitride layer, a two-metal oxide-nitride layer, or acombination thereof.
 6. The nonvolatile memory element of claim 5,wherein the metal of the metal oxide layer comprises one or more ofaluminum (Al), hafnium (Hf), zirconium (Zr), titanium (Ti), tantalum(Ta), nickel (Ni), copper (Cu), tungsten (W), iron (Fe), vanadium (V),niobium (Nb), or tungsten (W).
 7. The nonvolatile memory element ofclaim 5, wherein the resistive switching layer comprises at least one ormore of AlO_(x), HfO_(x), TiO_(x), AlN_(x), AlO_(x)N_(y), HfN_(x),ZrN_(x), SiN_(x), HfO_(x)N_(y), ZrO_(x)N_(y), AlO_(x)N_(y),TaO_(x)N_(y), Hf_(x)Si_(y)O_(z)N_((1-x-y-z)),Zr_(x)Si_(y)O_(z)N_((1-x-y-z)), Hf_(x)Zr_(y)O_(z)N_((1-x-y-z)), orHf_(x)Al_(y)O_(z)N_((1-x-y-z)).
 8. The nonvolatile memory element ofclaim 1, wherein the first layer comprises an electrode comprising aconductive metal.
 9. The nonvolatile memory element of claim 1, whereinthe second layer is a bottom electrode comprising one or more of p-typepolysilicon, n-type polysilicon, titanium, aluminum, or tungsten. 10.The nonvolatile memory element of claim 1, wherein the second sub-layerhas a thickness of about 5 nanometers (nm) or less.
 11. The nonvolatilememory element of claim 1, wherein the second sub-layer comprises ametal oxide layer, a two-metal oxide layer, a metal nitride layer, ametal oxide-nitride layer, a two-metal oxide-nitride layer, or acombination thereof.
 12. The nonvolatile memory element of claim 11,wherein the metal comprises one or more of aluminum (Al), hafnium (Hf),zirconium (Zr), titanium (Ti), tantalum (Ta), nickel (Ni), copper (Cu),tungsten (W), iron (Fe), vanadium (V), niobium (Nb), or tungsten (W).13. The nonvolatile memory element of claim 11, wherein the secondsub-layer comprises one or more of AlO_(x), HfO_(x), TiO_(x), AlN_(x),AlO_(x)N_(y), HfN_(x), ZrN_(x), SiN_(x), HfO_(x)N_(y), ZrO_(x)N_(y),AlO_(x)N_(y), TaO_(x)N_(y), Hf_(x)Si_(y)O_(z)N_((1-x-y-z)),Zr_(x)Si_(y)O_(z)N_((1-x-y-z)), Hf_(x)Zr_(y)O_(z)N_((1-x-y-z)), orHf_(x)Al_(y)O_(z)N_((1-x-y-z)).
 14. A method of forming a nonvolatilememory element in a ReRAM device, the method comprising: forming aresistive switching layer on a first layer; and forming a second layer,so that the resistive switching layer is disposed between the firstlayer and the second layer, wherein the resistive switching layercomprises a material having the same morphology as a top surface of thefirst layer; forming a first sub-layer on the first layer, the firstsub-layer comprising the material formed with the same morphology as atop surface of the first layer; and forming a second sub-layer on thefirst sub-layer, the second sub-layer comprising a material formed witha morphology that is different than the first sub-layer and the firstlayer, wherein the first layer and the first sub-layer have apolycrystalline morphology and the second sub-layer has a crystallinemorphology.
 15. The method of claim 14, wherein the morphology of thefirst layer and the resistive switching layer is crystalline,polycrystalline, amorphous, or intermixed.
 16. The method of claim 14,wherein the resistive switching layer has a thickness of about 5nanometers (nm) or less.